The present invention relates to the nondestructive determination of the composition of a material when comparing the thermal properties of a sample of the material with the thermal properties of a standard of a similar material, said standard or substitute having a desired composition.
The invention herein described has use for nondestructive qualitative determination of composition of a variety of materials and its use is discussed mostly with reference to precious metals, such as gold and silver.
The rise in trading in recent years of precious metals, such as gold and silver, as commodities, and the rise in their unit prices has increased the need for an economical, fail-safe mechanism for determining nondestructively, the purity of such materials. Since gold, like silver, is often transferred or sold by persons not particularly knowledgeable about such precious metals to one of greater knowledge, it is important that some way be found to detect forgeries and ascertain the purity of such precious metals that avoids the costly and time consuming methods, several of which are outlined below, a way that is nondestructive, fast and accurate.
By way of background, that which follows was paraphrased from The collector ""s Dictionary of the Silver and Gold of Great Britain and North America, Michael Clayton, World Publishing Company, 1971.
Pure gold is extremely heavy in proportion to its volume and also very soft (malleable), and if pure, is referred to as xe2x80x9824 caratxe2x80x99. Silver is not so malleable and only approximately half the weight of a piece of gold of similar volume. Both are too soft to use in their pure form and must be hardened by the admixture of base metals, usually copper, though silver may be used with gold. If both silver and copper are added to gold it becomes pale and green in color. The fact that adding a 50 percent alloy of copper to silver retains a silvery appearance can easily lead to fraud without protection. In general, the best proportions of gold and alloy are 22 parts pure gold to 2 parts alloy, but this can be varied so that 18, 15, 12, or 9 carats (or parts) are balanced by an alloy making up 24 parts. These balanced fractions of pure gold and alloys are legally used and obviously the less pure gold parts, the cheaper the finished product. With silver, only two standards are permitted in Great Britain, 925 parts out of 1000 as normal Sterling standard and 958.3 parts as the higher Britannia standard. By the same measurement, 18 carat gold is the equivalent of 750 parts fine gold to 1000 (24 carats). For most of the English gold coinage and all since 1672, the fineness has never been below 22 carats (916.66 parts gold to the 1000). Since the nineteenth century in the United States, the coin standard is 900 parts pure silver to 100 parts alloy.
In order to protect the buyer of gold and silver, a system of testing, or assaying, and checking the quality and standards of an object is necessary. This can be done by comparison (touch), weight, or chemical means. The first demands considerable visual skill as the object to be tested and a piece of known quality are both stroked across a piece of basanite, a hard flint-like slate, and the resulting streaks compared. In the second test, weight, small portions of the object to be assayed are scraped from each piece, wrapped in lead (lead and silver are also used to wrap gold) and heated in a bone-ash crucible. As the heat is applied lead and other base metals oxidize and are absorbed by the crucible, known as a xe2x80x98cupelxe2x80x99; the balance is then weighed and compared with the weight of the original scrapings.
In the case of gold, which is also wrapped in silver, a further process is required whereby the silver is finally removed by placing it in hot nitric acid. This method was first recorded in 1495. If on completion of these tests, the gold or the silver are found to be below the lowest permitted standards, the marks which would guarantee their quality, xe2x80x98hall-marksxe2x80x99, as they are known, are withheld and the objects under examination are crushed and returned to the maker. The third is a simple method and applicable only to silver, but requires some reasonable idea of the quality of the metal being tested. This involves the dissolving of the weighed scrapings, also known as xe2x80x98dietxe2x80x99, in nitric acid and the addition of a standard solution of sodium chloride (common salt); at a certain point the cloudy liquid clears and silver chloride is precipitated. A comparison of the original weight of the silver sample and the quantity of saline solution required to do this enables the fineness of the metal to be assessed.
Historically, as indicated above there are a number of methods used to determine the composition of metallic materials that can be classified as comparative as well as destructive. A comparative method is one, as the name implies, that requires a comparison to a known reference material. A destructive test is as the name implies and needs no explanation. The descriptions of pertinent testing methods that follows are all comparative tests and are categorized as xe2x80x98destructivexe2x80x99 or xe2x80x98nondestructivexe2x80x99. The following paragraphs, under the heading xe2x80x9cPrior Artxe2x80x9d, discuss appropriate examples of these.
Destructive Tests
Some of the more modem methods, than those described above, that have been developed and in use today to determine alloy content of metallic materials are: optical emission spectrography, spectrometry, x-ray fluorescence spectrometry, atomic absorption spectrometry, plasma emission spectrometry and combustometric analysis to determine particular elements. Such methods are not only costly, but usually require a sample from the test piece, and thus are somewhat destructive.
A primary example of a destructive test is the standard prescribed by the American Society for Testing and Materials, (ASTM) Test Method B 562-95, xe2x80x9cStandard Specification for Refined Goldxe2x80x9d. This test method examines samples taken from the melt before pouring the casting of gold. The standard utilizes, for 99.5 percent purity, a test method for chemical analysis by cupellation fire assay. If there is a disagreement between the manufacturer and the purchaser the specified test will then be in accordance with ASTM Test Method E 1446, xe2x80x9cTest method for Chemical Analysis of Refined Gold by Direct Current Plasma Emission Spectroscopyxe2x80x9d.
The standard for testing silver, which is also destructive, is that given by the ASTM Test Method B 413-89, xe2x80x9cStandard Specification for Refined Silverxe2x80x9d. This method requires that the samples be taken from bars by drilling six holes and the chemical composition is determined in accordance with ASTM Test Method E 378 xe2x80x9cTest Method for Spectrographic Analysis of Silver by the Powder Techniquexe2x80x9d.
Portable electronic gold testers that measure the carat value of gold are also available, such as those described in U.S. Pat. Nos. 4,799,999 and 5,218,303, authored by Medvinsky and Radomyshelsky. These patents describe a method for determining the assay of gold alloy, utilizing an electrochemical process. The specimen gold is wetted by an electrolyte, and a small current anodizes the surface of the specimen for a metered period of time. A potential sensing device is then applied to the charged surface, and a potential decay is observed. The potential decay information is compared with empirical data and by interpolating the potential with the empirical data a determination of the carat quality of the gold alloy may be determined. This same method may be used for other precious metals, employing different electrolytes, empirical standards, and potentiometers.
There are two additional patents, U.S. Pat. Nos. 5,128,016 and 5,080,766, authored by Moment and Nelson, that essentially utilize the same technique with some variation as those indicated above.
Criticisms of these gold testing devices are that they are slightly destructive, are surface sensitive only, will not detect plating or gold overlay, and will leave a mark on items that are of 14 carat or less.
Nondestructive Tests
There are several methods of nondestructively discriminating between bodies having similar appearances but of slightly different composition or even of different material. In one instance the relatively old technique of eddy current testing is utilized to attempt to separate higher grade from lower grade materials. This method principally compares the subsurface electrical conductivity, synonymous with thermal conductivity, and magnetic permeability of a resulting read-out waveform of the higher grade standard material to that of the sample. The conductivity of gold and a mixture of gold with an adulterant will be very similar, as will silver and a mixture of silver with an adulterant, and thus the sensitivity of the eddy current technique will not be sufficient to separate such forgeries. Also, if a tungsten body, which has the same density as gold, is gold plated at a surface depth deeper than the subsurface penetration of the eddy current, then this test method will not discriminate between pure gold and the forgery.
In another instance U.S. Pat. No. 4,255,962, issued to Ashman, teaches a method of distinguishing a simulated diamond from a natural diamond by utilizing a probe which applies a pulse of heat to the surface of the sample in an air environment and during the occurrence of thermal equilibrium the same probe detects the change in temperature. This change in temperature is related to the thermal conductivity of the sample. Since the thermal conductivity of natural diamond is at least an order of magnitude greater than a simulated diamond, such as cubic zirconia, it is readily detected. This method, however, is not sensitive enough to detect the slight change in thermal conductivity between pure gold and a forgery or pure silver and a forgery.
Another example of a nondestructive test method is described in U.S. Pat. No. 3,981,175, which was issued to Hammond, III and Baratta. In accordance with that patent, the device is a nondestructive counterfeit gold bar and silver bar detection system based upon heat transfer principles. Regarding the testing of gold the principle entails the application of identical finite suddenly applied controlled heat pulses at a first region which is one end of an elongated gold bar of specific dimensions and of known purity, used as a standard, and a geometrically identical test bar. The system is completely enclosed in an insulating medium. The temperatures, which are measured at a second region at the far end of each bar are not only dependent upon the thermal properties of each bar, but upon its length and the length of the test time. Those thermal properties, which in gold are unique, are specific heat, thermal conductivity and density; the combination of these properties is defined as thermal diffusivity. Since these properties in gold are singular, the temperature at the second region, i.e., the end opposite from that which is suddenly pulsed by a quantity of heat, will usually be at a higher temperature in a given time than that of a bar of a particular length less pure than the standard gold bar of the same length. Because of the large differences in thermal properties of gold and an alloyed gold sample, temperature measurements conducted at the far end will reveal differences.
The general heat transfer equation for the aforementioned situation is given in the following:
If heat (e.g., a square wave pulse of indefinite duration) is applied to one end of a gold bar, at x=L, the general equation given in U.S. Pat. No. 3,981,175 for the temperature T(x,t) at any distance x along the bar""s length is:
T(x,t)=QL/k{xcex1t/L2+(3x2xe2x88x92L2)/6L2xe2x88x922/xcfx802xcexa3m=1∞(xe2x88x921)m/m2[exp(xe2x88x92xcex1m2xcfx802t/L2)]cos(mxcfx80x/L)}xe2x80x83xe2x80x83(1)
Where: Q is the suddenly applied constant heat flux applied over an area (BTU/sec-ft2) of the bar, at x=L, L is length in feet, k is the thermal conductivity (BTU/sec-ft-F), xcex1=k/xcfx81c, which is the thermal diffusivity in ft2/sec, c is the specific heat (BTU/lb-F), xcfx81 is the density in lbs/ft3, t is time in seconds, x is the distance in feet along the length of the sample and T(x,t) is temperature in degrees F. Note at x =0, at the far end there is no flow of heat because of the insulation, See Carslaw and Jaeger Conduction of Heat in Solids, Oxford Press, 1950.
The nondestructive testing of silver bars described in U.S. Pat. No. 3,981,175 is essentially the same as that indicated above except rather than employing a pulse of heat a constant temperature source is applied. Silver has the highest thermal diffusivity of any known material and the equation for the temperature along the bar length is dependent upon thermal diffusivity. Therefore, as a function of time, the silver bar will attain a higher far end temperature than any other material. The equation for the temperature at the far end is given in the following:
T(t)=2T0xcexa3n=0∞(xe2x88x921)n{1xe2x88x92erf[(2n+1)/2(xcex1t/L2)xc2xd]}xe2x80x83xe2x80x83(2)
Where T0 is the applied constant temperature above ambient and xe2x80x98erfxe2x80x99 is the standard definition of the error function; well tabulated in many references.
The method of U.S. Pat. No. 3,981,175 requires that the standard and test sample be completely insulated and the further restrictions are that: The standard and test sample must be elongated bars of the same particular length, and temperatures at the far end of each bar must be taken over same particular time interval after the heat is applied, depending on the length of the bar.
Yet another example of a nondestructive test method to detect fraudulent precious metal bars is revealed in U.S. Pat. No. 4,381,154, issued to Hammond, III. It was found that of all possible forgeries, a non-alloyed tungsten forgery of gold, i.e., an insert of tungsten within the gold bar, is the most difficult to detect because the density and heat-capacity of tungsten and gold are virtually identical (a less difficult forgery to detect is an alloyed forgery wherein its composition is generally uniform throughout). Thus, an improvement in accuracy over the previous U.S. Pat. No. 3,981,175 was required at that time. This improvement consists mainly of increasing the accuracy of the detection system by providing and controlling heat into the test chamber resulting in equilibrium, termed xe2x80x9cdynamic insulationxe2x80x9d by the author; accurate heater control and using a compensated infrared sensor to measure the temperature at the far end opposite the heated end of the sample. Also the author claimed that this method allowed the determination of the density, thermal conductivity and heat capacity of a given material.
Although the improved techniques adopted in U.S. Pat. No. 4,381,154 will enhance the sensitivity of this test method, it still requires that the test piece be an elongated bar of specific dimensions and additional temperature sensors, controls and electronic instrumentation compared to the method of prescribed in U.S. Pat. No. 3,981,175. It is also noted that the present day infrared temperature sensors can readily determine temperatures to an accuracy within 0.10 C. over a wide range of temperatures (see the paper by J. M. Looney, JR., and F. Pompei, Medical Electronics, 1989), thus superseding the method proposed in U.S. Pat. No. 4,381,154.
An additional improvement is described in U.S. Pat. No. 4,385,843 granted to Hammond, III, whereby an induction heater is employed to provide a pulse of heat to a bar of precious metal to determine if it has the purity of composition within a given range of variance. Heat is induced at one end of the bar using an induction heater powered by a high frequency power source, and the time versus temperature response at the other end of the bar is monitored. This device was employed, according to the author, to circumvent the problems associated with contact heaters. However, present day lasers or infrared heat sources will accomplish the same goal.
A more recent U.S. Pat. No. 5,052,819, was issued to Baratta; this document taught a method of nondestructively identifying materials and fraudulent carbon steel fasteners. This invention compared the characteristic temperature-time curve of a standard fastener to a test fastener by simultaneously providing a pulse of heat to both fasteners and measuring the temperatures at their heated ends. However this patent required an insulated receptacle and specified that the standard and test sample be restricted to elongated bars.
The device will provide, broadly, a method of determining nondestructively, the purity or composition of an unknown material sample, such as, for example a sample of gold or silver of unknown purity. The form of the sample can be a casting, a bullion, a coupon or a disc (a coin) or even gold or silver rings with partially flat surfaces, such as signets. The uniqueness of the invention involves first subjecting one of the large surfaces of the sample of known thickness to a constant energy heat pulse or a constant cold pulse relative to the initial temperature of the sample and comparing the time-varying temperature pattern at the same surface thereof, or at the opposite surface during finite lengths of time with that of a known and identically-sized standard subjected to the equivalent conditions for an interval of time of the same finite length. The temperature of said surface or the opposite surface can be monitored during the time the heat or cold pulse is applied and/or after withdrawal. The second test, if needed, is in the form of the application of constant temperature and will eliminate an adulterated gold item that may not have been detected by the first test; this is referred to as a dual test method. In addition, the slopes of the time-varying temperature patterns during the time the condition is applied and/or after it is withdrawn can also be determined.
Improvements over the present state-of-the art consist of eliminating the need for a specified sample shape such as an elongated bar of a particular length, as well as a completely insulated environment and allowing testing of samples whose surfaces are exposed to a medium, and the use of both contacting and non-contacting heating units; and noncontacting temperature sensing elements. Further improvements are realized by examining: the slope of the temperature-time curves, the decay of the temperature-time curves after the heat or cold pulse, or constant temperature is removed, as well as the slope of the decay curve. These improvements are applicable to field operations.